The present invention relates to a corona discharger of the type using a wire and applicable to an electrophotographic copier or similar apparatus which effects an electrostatic process so as to serve as a charger for charging a photoconductive element, a discharger for discharging it, or a precleaning discharger of a cleaning unit.
A corona discharging device of the type described is extensively used with an electrophotographic copier which performs an electrostatic process for the purpose of charging or discharging the surface of a photoconductive element. A prior art corona discharging device of this type is generally made up of a gutter-like shield case, end blocks provided at opposite sides of the end block and made of an insulating material, and a corona wire connected to a feed terminal by a spring which is disposed in one of the end blocks and extending between the end blocks. When a high voltage is applied from the feed terminal to the corona wire via the spring, corona discharge occurs between the wire and the shield case to charge or discharge a photoconductive element which is located in the vicinity of an opening of the shield case.
A problem with a corona discharging device of the type described is the leak of current which occurs at the end blocks. Specifically, the leak occurs between the spring or the corona wire which is disposed in each end block and a grounding member adapted to ground the shield case, sometimes over the space between the spring or the corona wire and the grounding member and sometimes along the surface of the end block. The leak over the space and the lead along the end block surface will hereinafter be referred to as spatial leak and creeping leak, respectively. The leak disturbs corona discharge and thereby the quality of a reproduction. In addition, a control circuit is apt to malfunction due to electrical noise which is ascribable to leak current. Implementations customarily adopted against the leak are as follows:
(1) increasing the distance between the spring in the end block and the grounding member;
(2) forming undulation on the surface of the end block to increase the creeping distance between the spring and the grounding member; and
(3) disposing a screening member made of an insulating material between the spring and the grounding member.
The approach (1) stated above is allowable if an apparatus in which the corona discharging device is incorporated has a relatively large size. When the size of such an apparatus is relatively small, the approach (2) or (3) is adopted because a sufficient distance is not available between the spring and the grounding member. Especially, the screening member in accordance with the approach (3) is effective when constituted by a cover which closes the opening of the end block. This is because the end block is necessarily provided with an opening for laying and replacing the corona wire. However, since the prior art end block cover closes the opening of the end block from one side only, gaps are not avoidable between the interengaging surfaces of the end block and cover although they may be invisible, however accurate the fininishing of those members may be. It follows that the spatial distance or the creeping distance between the grounding member and the spring disposed in the end block cannot be increased beyond a certain limit. Further, it is difficult to provide undulation on the end block due to the current trend toward the miniaturization of apparatuses of interest.